1. Field of the Invention
This invention relates to fatigue monitors and more particularly to a novel and highly effective device for detecting fatigue of a monitored structure due to flexure and to a method of applying the device to the monitored structure. The invention is applicable particularly to the detection of metal fatigue but is applicable also to the detection of fatigue in other structural materials.
2. Description of the Prior Art
U.S. patents to Maurice A. Brull U.S. Pat. No. 4,590,804 and U.S. Pat. No. 4,639,997, both assigned to the assignee of the present invention, disclose respectively a device for and method of monitoring fatigue life. The patents illustrate what are called coupons, otherwise known as fatigue fuses, arranged in a row and each having different notch patterns ranging from the mildest notch pattern to the most aggressive notch pattern. The coupons will fail because of fatigue in a prescribed sequence. The number of load cycles to failure as a function of stress amplitude for the different coupons can be plotted, and as each coupon fails, it gives an indication of the remaining life expectancy of the monitored structure.
More particularly, each of the coupons includes a special notch pattern comprising at least one pair of notches designed to produce a local stress concentration. One notch of each of the notch pairs is disposed on each of the longitudinal sides of the coupon, the notches of the notch pair being substantially geometrically the same. Their axis must be oriented along a suitably chosen direction. The notch pattern of each of the coupons produces a stress field which varies in intensity from relatively mild to very severe. The severity of the local stress field is controlled by the geometry of the notch pattern. Smooth geometries produce a mild stress concentration, while geometric discontinuities produce severe stresses. In this manner, if all of the coupons can be subjected to the same stress history, it will result in the development of different stress concentrations in the region of the notch tips of each coupon, so that each coupon will have a different fatigue life. Moreover, if the stress history of the coupons is the same as that of the monitored structure, the fatigue life of each coupon will be a different predetermined percentage of the fatigue life of the monitored structure.
In accordance with the disclosures of the patents, the coupons are secured to the monitored structure by pins, adhesive or spot welding so that, ideally, all of the coupons experience the same strain history as the monitored structure.
It has now been discovered that the mounting techniques of the prior art, while producing good results in certain circumstances, fail to produce optimum results under other circumstances. In particular, the mounting techniques of the prior art fail to ensure that the coupons experience the same stress history as the monitored structure. The discrepancy between the stress history of the coupons and that of the monitored structure is particularly large in the case of compressive stresses.
This has practical consequences because, in many typical cases, the monitored structure will undergo stresses that alternately produce tension and compression, as illustrated in FIG. 1.
If the fuse is not subjected to compressive stresses to the same extent as the monitored structure, the fuse will not fail when it ideally should in view of the predetermined notches formed in it. In response to a compressive stress, a given fuse mounted in accordance with the prior art may bend out or in at the notches and will thus fail unpredictably. A coupon designed to fail at, say, 50% of the fatigue life of the monitored structure may actually survive intact until failure of the monitored structure, thereby giving no warning at all.
Moreover, the prior fuse is not adapted for incorporation in a remote-monitoring system. Remote monitoring is very important where the monitored structure is not readily accessible for inspection. In modern aircraft, for example, there are vital structural components deeply recessed in cavities that are stuffed with electrical leads and components, hydraulic lines, and other elements that render access for visual inspection time-consuming, difficult and expensive.